The present invention relates to a diffractive optical element, and in particular, to a diffractive optical element having a structure in which diffraction gratings formed of materials different from each other are stacked.
Methods for reducing chromatic aberration in a lens system include one in which a diffractive optical element is provided to a surface of a lens or part of the lens system. This method using the diffractive optical element utilizes a physical phenomenon that chromatic aberrations for a light ray at a certain reference wavelength are generated in directions opposite to one another by a refractive surface and a diffractive surface in an optical system. Further, the diffractive optical element is capable of providing an effect as an aspheric surface lens by varying a period of its periodic structure, which brings about an effect of reducing not only the chromatic aberration, but also aberrations other than the chromatic aberration.
In a case in which, in an optical system including the diffractive optical element, a light flux in a use wavelength range mainly includes diffracted light of a certain specific order (hereinafter referred to as “specific order” or “designed order”), intensities of diffracted lights of orders other than the designed order are low. The intensity 0 means nonexistent diffracted light.
However, in reality, in a case in which the unnecessary diffracted lights of the orders other than the designed order exist and they have high intensities, the unnecessary diffracted lights travel through the optical system along paths different from that of the diffracted light of the designed order to become flare light. Therefore, in order to utilize the aberration reduction effect of the diffractive optical element, diffraction efficiency for the diffracted light of the designed order is required to be sufficiently high in the entire use wavelength range. Further, it is important that a spectral distribution of the diffraction efficiency of the diffracted light of the designed order and behaviors of the unnecessary diffracted lights of the orders other than the designed order are sufficiently considered.
Japanese Patent No. 3717555, Japanese Patent Laid-Open No. 11-271513, Japanese Patent Laid-Open No. 2003-227913, and Japanese Patent Laid-Open No. 2005-107298 (hereinafter respectively referred to as Documents I to IV) disclose diffractive optical elements having a structure in which its diffraction efficiency is improved to reduce unnecessary diffracted light. Each of the diffractive optical elements disclosed in Documents I to IV is a diffractive optical element in which two diffraction gratings are stacked so as not to provide any air layer (or gap, or space) therebetween. Such a diffractive optical element is hereinafter referred to as a “contacting two-layer DOE”. In Documents I to IV, a diffractive optical element is realized which has a tolerably high diffraction efficiency for diffracted light of a specific order in a broad wavelength range by appropriately setting materials forming the respective diffraction gratings and heights of the grating portions (grating heights) respectively having slope surfaces and side surfaces. The diffraction efficiency is expressed by a percentage of an amount of diffracted light of each order with respect to an amount of an entire transmitted light flux.
However, in Documents I to IV, only an effect of the slope surfaces of the diffraction gratings is taken into consideration, but an effect of the side surfaces of the diffraction gratings is not taken into consideration. Therefore, the diffraction efficiency for the specific order (designed order) is not sufficiently high.